The present invention relates to welding electrodes, and to a process for forming same.
Resistance welding has long been used as a quick and effective method of joining metal members. The workpieces to be welded are placed in abutting relationship and a large electrical current is caused to flow through the workpieces by a pair of opposed electrodes that contact the workpieces on opposite sides of the weld point. The current causes the abutting surfaces of the workpieces to be heated sufficiently to effect the formation of a weld nugget. Typically, the electrodes apply significant pressure to the workpieces during welding. This facilitates the welding process by urging the material together and, also, reducing electrical resistance between each electrode tip and the adjacent workpiece material that it contacts.
Since welding is accomplished by resistance heating of the material being welded, it will be appreciated that the electrodes will also be heated substantially. It is important to have electrodes of high electrical conductivity in order to minimize the power loss in the electrode, and the resulting heating of the electrode.
Over time, the repeated heating and pressing operations involved in resistance welding cause breakdown, softening, mushrooming and other deformation of the electrodes. The current density of the current going through the workpieces drops. As this occurs, electrical current requirements for welding increase with the enlarged welding tip face contacting the workpiece material until ultimately, redressing or replacement of the electrode is required. Accordingly, it is also important to have an electrode which is capable of withstanding significant distorting force at the elevated temperatures which result from the welding process so as to minimize the number of times it becomes necessary to redress or replace the electrode within a given period of time.
It is known in the art to form resistance welding electrodes by combining a copper electrode body with an anneal resistant, high hardness insert. Typically, the insert performs much better than the copper material from which the electrode body is formed. However, the insert material is much more expensive than the copper used to form the electrode body.
The insert may be brazed onto the shank of the electrode. The brazing step is disadvantageous, however, as it adds an additional step to the electrode manufacturing process and, hence, increases the cost of the electrode. Furthermore, the brazing operation may anneal and soften the electrode body.
It is also known to force the insert into an electrode body via a press-fit operation. The steel being welded today is often galvanized or coated with a zinc or other, softer metal coating. As a result, resistance welding electrode may tend to stick to the coated metal. An electrode tip joined to an electrode body only by means of a press-fit may tend to pull out of the body as the electrode is retracted following resistance welding of coated materials, thus requiring replacement of the electrode.
Accordingly, there is a need for an improved resistance welding electrode which can be manufactured via an efficient and more cost effective process and, yet, is capable of performing in an acceptable manner.